Accessibility - accessibility forces you to look at your potential web audience as a whole in order to customize content and web information for everyone, universally, regardless of impairment/disability (this pertains to physical as well as technical). There are many things to consider such as ensuring sure that 1)text can be re sized 2) forms are accessible and user friendly 3) having written transcripts for the hearing impaired 4) designing your website for those whose browsers do have support images by using alternative text 5) ensuring that the "shift" and "tab" keys can be used throughout your website (for those that do not have use of a mouse). Obviously, these are just a few checkpoints to think about on the technical side when designing a webpage. There are free programs on the Internet that can help you check your site for accessibility (Bobby and Wave).
Usability - making sure that yours site is "usable" and easy to use/user friendly. Your site should be easy enough to navigate through without making the user think too much. Awesome. I think I can do that part! Personally, as much as I love the Internet, if a site is not easy to use, regardless of how pretty the graphics and colors are, I will "x" out of it immediately.
It's awesome to be able to have the freedom to create your own website whether it's for commercial or personal purposes. The advantages/disadvantages of creating your own website as opposed to getting someone else to do it for you basically boils down to time, money and vision. Obviously, it is cheaper to create your own website but your website could suffer if you are not tuned into all of the technical information that is needed to make your website a success. On the other hand, if money is more of a friend to you than time is, it would be well worth it to hire someone to do this for you (providing that they are reputable and are able to clearly communicate and incorporate your personal vision and passion into your website - if not - well that's another page to incorporate in the Cumbersome WebSite Designing Process).
It's nice to have options right? As Michael Jackson would squeal "HE HEEEE"!!! R.I.P. Michael.